Multicasting or sharing a transmitted signal with information among a group of receivers who are interested in the same content (e.g. data, video, audio, etc.) is generally an effective and scalable way to deliver bandwidth-intensive data in wireless or even wired infrastructure because duplicated deliveries of the same copy of data can be prevented.
Unprecedented advancements in wireless broadband access technologies based on IEEE 802.16 (WiMAX) standards and scalable video coding technologies, such as H.264/MPEG4 Advanced Video Coding (AVC), have made it possible for provisioning large-scale wireless video multicast/broadcast services, such as mobile Internet Protocol Television (IPTV). It is accepted that adopting video multicasting achieves the best scalable usage of transmission capacity, where system resource allocation only concerns the number of TV channels under provisioning along with their bandwidth requirements instead of the number of recipients. This facilitates large-scale and high quality wireless multicasting and broadcasting for video data including scheduled and live TV content, in which multiple receivers are expected to simultaneously receive the bandwidth-intensive data of the same video stream.
However, due to multi-user channel diversity among subscribers in receiving the same wireless multicast signal, it has long been an issue how to select a proper transmission rate. A mono-rate multicast signal could be under-utilizing the channels capability of some receivers with good channel conditions while not decodable by some receivers with bad channel conditions. A straightforward yet dummy solution could be such that the most conservative transmission rate is adopted to seek to satisfy as many recipients as possible, at the expense of a reduced number of TV channels that can be jointly provisioned, which certainly leads to a poor economic scale. Superposition coding (SPC) is a physical layer technique that allows a transmitter to send individual information to multiple receivers simultaneously within a single layered wireless broadcast signal. A SPC signal contains multi-resolution modulated symbols, which enable a receiver to decode its own, as well as its peers', information depending on its channel condition at the instant of decoding. SPC can be employed in a cross-layer design to form the wireless multicast signals for transmitting scalable video bitstreams with multiple quality layers for IPTV services in broadband wireless access (BWA) networks, such as WiMAX. Such a cross-layer design framework can effectively tackle the multi-user channel diversity problem, and the generated multi-resolution modulated signal can scale the multicasting/broadcasting of common successively refined information like scalable video bitstreams. By superimposing multiple quality layer information into a single SPC modulated signal, the receivers with poor channels can decode and obtain the base layer data to achieve basic video perceptual quality, while the receivers with good channel conditions may obtain the data of higher quality layer(s) which refines the data of lower layer(s), in order to yield improved visual perception of video quality.
SPC provides a means by which two or more independent receivers' information may be transmitted to the receivers by superimposing the modulated signals corresponding to each receiver into one single signal. Such superposition of multiple signals could be understood by way of vector addition. SPC can be used, for example, to provide superposition of two or more signals.
FIG. 1 illustrates a vector addition corresponding to SPC of two signals for two receivers, in which x1 with information for receiver 1 is modulated by QPSK for a faster throughput yet a higher channel requirement (i.e. higher SNR requirement), and x2 with information for receiver 2 is modulated by BPSK for a lower channel requirement (i.e. lower SNR requirement) but slower throughput under the same error rate. The superimposed signal, x, is a vector sum of the 2 receivers modulated signals governed by x=x1+x2. As illustrated in FIG. 1(a), vector x represents the superimposed signal, consisting of symbol ‘0’ and symbol ‘01’.
The signal x is then launched as a single wireless transmission block and received by two receivers with diverse channel conditions within the same coverage. The received signal may be expressed as yi=x+zi, where zi is the noise perceived by receiver i. The conventional decoding technique, known as Signal-Interference Cancellation (SIC), is typically used at receiver i to identify the signal components meant for the other receivers. Receiver i obtains its own information by subtracting those signal components meant for others from its received signal yi. For example, for receiver 2 to decode its data from y2, it must first use SIC to determine the data meant for receiver 1, x1, and then subtract x1 from the received signal y2. The result of the subtraction using SIC is aiming for x2, which is, usually, distorted by the noise experienced at receiver 2, i.e., z2. A schematic representation of these encoding and decoding processes is shown in FIGS. 1(a) and 1(b), respectively, for an intuitive understanding by considering negligible noises.
It can be comprehended that SPC may be optimally used for multicasting successively refined information, such as scalable (layered) video coded bitstreams, instead of independent information. By adopting SPC for scalable video coded bitstreams, a receiver can obtain the data of the base video quality modulated with a slower throughput but lower channel requirement when its channel condition is poor, but another receiver with good channel condition may obtain the full video quality by decoding the data of both base and enhancement video layers since it may be able to demodulate even the signal modulated by the higher throughput modulation scheme.
FIG. 2 illustrates a schematic diagram for the SPC multicast of BPSK and QPSK for a two-layered successively refined video source. The use of SPC modulation enables multi-resolution transmission rates to maximize quality perceived under good channel conditions yet still secure the conservative rate provided for the base layer from the same SPC multicast signal. Those skilled in the art are aware of the effectiveness of using superposition coding in video multicast over wireless channel in order to overcome the multi-user channel diversity.
In spite of the aforementioned advantageous features, few commercially available wireless systems and industry standards related to wireless video multicast have adopted the SPC modulation. The absence of SPC modulation in video multicast is likely attributed to the requirement of additional system support, in which dedicated hardware components and circuitry are needed to superimpose two or multiple modulated signals together to form a SPC signal in the PHY layer. Also, some software modifications are required for enabling the cross-layer mapping between the successively refined video source and the layered modulation by SPC. Current 3G technologies and previous wireless systems failed to justify such an addition of dedicated hardware and software support mostly due to the fact that video multicasting service subscriptions, such as scheduled IPTV, have not reached maturity. These requirements pose both a significant technical barrier and interoperability concerns in adopting and deploying this standard approach for scalable wireless video multicasting in broadband wireless access (BWA) networks.
By envisioning the prevalence of bandwidth demanding video multicasting services provisioned on the emerging BWA networks, it is becoming crucial to define and position a practical implementation of SPC video multicasting that offers the minimal barrier for industry acceptance.
Therefore, what is required is a new design architecture for SPC that mitigates the effect of multi-user channel diversity but that can be used with minimal modifications to existing wireless transmitters and preferably without any hardware modifications to existing wireless receivers.